The human eye is like a camera that collects, focuses, and transmits light through a lens to create an image of its surroundings. In a camera, the image is created on film or an image sensor. In the eye, the image is created on the retina, a thin layer of light-sensitive tissue at the back of the eye. When light enters the eye, photoreceptors in the retina absorb the light rays falling on them and convert their energy into electrical impulses, which then travel along the optic nerve to the brain where they are interpreted into visual images. When the photoreceptors in the retina are diseased or damaged, severe or total loss of vision (visual impairment) can occur. Nearly 10,000,000 people around the world suffer from some sort of visual impairment or handicap due to photoreceptor damage.
Until recently, those affected with a visual impairment were left without hope of a cure or even a treatment that would somewhat improve their vision. However, over the last few years, visual neuroprostheses, artificial devices which are inserted in the eye behind or in front of the damaged retinal area, have become available. Electrical stimulation of almost any location along the visual path can evoke phenomenon of perceived vision characterized by “phosphenes”. Although the first attempts to restore vision in blind patients date back to the early 20th century, the first mainstream attempts at visual neuroprosthetics took place in the mid-1950s when G. E. Tassiker invented a light-sensitive selenium cell. This cell was to be placed behind the retina of the blind patient and its purpose was to give them the perception of bright sensations. In the 1960s and 1970s, scientists attempted to restore vision by placing electrodes directly onto the surface of the visual cortex. Unfortunately, these implants did not work because they did not provide any useful images. In the 1990s, scientists switched to the idea of the photoreceptive chip. These photoreceptive chips, in theory, should provide information to the healthy neurons residing in the retina, substituting for the damaged photoreceptors. Known devices typically employ arrays of stimulating electrodes powered by photodiodes or microphotodiodes (components that produce an electrical current, voltage potential, or electrochemical potential in response to light) disposed on the epiretinal side (the surface of the retina facing the vitreous cavity) or the subretinal side (the underneath side) of the retina.
There are many limitations with current photoreceptive chips. First, the number of electrodes that come into contact with the neural tissue or ganglion/horizontal cells is too small to function as the tissue naturally would. Second, the electrodes used in the implants are prone to rejection and they tend to decline in performance over time. Thus there is a need in the art for a photoreceptive chip that is biocompatible, flexible, and mimics the natural photoreceptor density of the retina, thus providing a high level of visual acuity.